Fire resistance of high-strength circular steel-tube-confined steel-reinforced concrete columns under eccentric compression

This paper presents a systematic experimental and numerical study on the structural behaviour and fire resistance of high-strength circular steel-tube-confined steel-reinforced concrete (HCST-SRC) columns under eccentric compression. Eight specimens were tested at ambient and elevated temperatures, considering steel reinforcement yield strength, load ratio, and eccentricity. At ambient temperature, all specimens underwent global flexural failure, with satisfactory composite action between the outer steel tube and internal steel reinforcement and without significant local buckling. The load-carrying capacity was more significantly influenced by eccentricity and slenderness ratio than by steel reinforcement yield strength. An increase in eccentricity from 25 mm to 50 mm resulted in a 31.7% reduction in ultimate capacity. The fire tests demonstrated that all specimens failed by global flexural buckling, with the load ratio being the most critical factor governing fire resistance. An increase in load ratio from 0.3 to 0.5 reduced the fire resistance from 128 min to 49 min, while the longer fire resistance observed at larger eccentricity was primarily associated with the reduced absolute load resulting from load-ratio normalization rather than an intrinsic beneficial effect of eccentricity. Parametric analysis further revealed a non-monotonic effect of eccentricity on fire resistance. A finite element model was developed and validated against test results, followed by parametric analysis. The results indicate that increasing cross-sectional dimension, steel reinforcement yield strength, concrete strength, and steel reinforcement ratio enhances fire resistance, whereas increasing slenderness ratio, steel tube strength, and steel tube area ratio exhibit adverse effects; the influence of load eccentricity on fire resistance is non-monotonic. A simplified formula for predicting fire resistance was proposed, and the predicted results are in good agreement with the finite element results, with absolute values of relative errors generally within 15%.